The present invention relates to an improvement in a portion of an ink ejection head which connects a nozzle hole and an ink chamber to each other.
In a charge and deflection type ink jet printing apparatus, ink is fed under pressure to an ink ejection head and ejected from a nozzle hole thereof as a jet of ink. The ink jet is separated into a string of droplets at a predetermined position and selectively charged and deflected to print out data on a sheet of paper. It is a prerequisite in this type of printer that the ejection of ink be sharply cut off in response to an interruption command. If any transient state occurs in which the intensity of the ink jet is slowly run down, the ink droplets will be simply dropped to smear a deflection electrode and/or charging electrodes located in front of the nozzle hole, damaging the printer in the worst case. Moreover, should the neighborhood of the nozzle hole be smeared by the ink which drips along the head, the direction of ink ejection would be effected to result in inaccurate reproduction of data. It has been customary to construct an ink ejection head such that, as soon as a solenoid operated valve for ink supply is closed, vacuum momentarily develops in an ink chamber inside the head in order to immediately cut off the ink ejection. Thus, the vacuum in the ink chamber sucks ink from the neighborhood of the nozzle hole into the ink chamber thereby interrupting the ink ejection instantaneously.
However, not only the ink but air are sucked into the ink chamber through the nozzle hole so that bubbles are allowed to stay in the ink chamber. If the bubbles remain a long time in the ink chamber even after the start of the next ink ejection, the bubbles create an undesirable damper which lowers the transmission efficiency of vibration necessary for separating a jet of ink into droplets. The lowered transmission efficiency dislocates the predetermined position where the jet is to separate into droplets and, thereby, causes the charging efficiency to fluctuate. Then, the amount of deflection of a charged ink droplet would be varied to disturb an image reproduced on a sheet of paper. It is important, therefore, to promptly dissipate the bubbles drawn into the ink chamber when ink ejection is resumed after a certain period of interruption.